1. Field of the Invention
The present invention generally relates to a system that may be used to remove multiple plugs from a wellbore. Specifically, the system of the present disclosure is efficient in fluidizing and circulating proppant located below a portion of an upper plug that rests on a proppant that has settled on top of a lower plug. The proppant causes the partially milled upper plug to spin within the wellbore as the mill turns. The system uses a central port in a mill to fluidize and circulate the settled proppant around the perimeter of the upper plug until the upper plug is able to mate and rotationally lock with a lower plug set within the wellbore. Upon locking, the system is able to rapidly mill out the remaining portion of the upper plug and mill out the lower plug until the lower plug is no longer set and drops down the wellbore. The mill of the disclosed system is adapted to jet fluid through a central opening of a portion of the upper plug to fluidize and circulate the proppant from beneath the partially milled upper plug. The system provides for the rapid removal of multiple plugs positioned within a wellbore wherein proppant is present between the plugs. Having the benefit of this disclosure, one of ordinary skill in the art will appreciate that the disclosed invention may be used to remove various types of plugs used to hydraulically isolate a zone within a wellbore in addition to bridge plugs referenced below.
2. Description of the Related Art
Perforating and fracturing a well is common practice in the oil and gas industry in an effort to stimulate the well and increase the production of hydrocarbons. After the casing in a zone of interest has been perforated, the zone of interest typically needs to be hydraulically isolated from lower zones before the zone is fractured. Typically, a zone is isolated by the insertion and setting of a plug, hereinafter referred to as a bridge plug, below the zone of interest. The purpose of the bridge plug is simply to hydraulically isolate that portion of the well from a lower portion (or the rest) of the well. The isolation of the zone limits high pressure fracturing fluid pumped into the well to the zone of interest. The high pressure fracturing fluid is used to fracture the formation at the perforations through the casing. The high pressure of the fracturing fluid propagates a fracture in the formation, which may increase the production of hydrocarbons from that zone of the wellbore. Fracturing fluid typically contains a proppant that aids in holding the fractures open after the fracturing process has been completed.
In many situations, the process of perforating the casing and isolating the zone of interest is repeated at multiple locations. A bridge plug is typically set within the wellbore to define the lower portion of each zone that is to be stimulated. At the conclusion of the perforating and fracturing procedure, each of the bridge plugs set within the wellbore may need to be milled out. In an attempt to reduce the overall time required to mill out the bridge plugs, there have been many improvements made to the design of bridge plugs in an effort to make the plugs easier to mill out.
For example, the material of the bridge plug can affect the milling time needed to remove the bridge plug from the wellbore. Bridge plugs used to be comprised of a material such as cast iron, which is a brittle metal, but is not easy to drill through using a milling assembly run on coiled tubing. Coiled tubing does not provide as much of a set down weight as prior milling assemblies that used jointed pipes. As a result, bridge plugs are now often comprised of generally softer, nonmetallic components so that they can be drilled quickly. Composite bridge plugs are now widely used and help to decrease the mill out time. The composite bridge plugs also make it easier to circulate bridge plug particles out of the wellbore than the prior cast iron bridge plugs.
Another potential problem with past drillable bridge plugs is the rotation of the bridge plug or the rotation of components within the bridge plug. Rotation of the bridge plug increases the mill-out time as would be appreciated by one of ordinary skill in the art. As a result the bridge plugs often include some sort of locking mechanism to prevent the rotation of components. Further, the anchoring assembly of the bridge plug helps to prevent the rotation within the wellbore. An anchoring assembly typically includes a plurality of slips and a cone, as well as an elastomeric packing element. However, once the mill has milled out the lower slips of the anchoring assembly, the remainder of the plug falls down the wellbore landing on top of the next bridge plug.
In the past, the remainder of a bridge plug located on the top of lower bridge plug presented another potential problem. Specifically, the partially milled out plug was able to rotate (i.e., spin) on top of the set plug, which again increased the milling time. Present bridge plugs have been designed to prevent such rotation. The lower portion of a bridge plug often includes a profile that is adapted to engage a corresponding profile on the upper portion of a bridge plug. When the lower portion of a bridge plug lands on a set bridge plug the upper bridge plug rotates until the two profiles engage creating a rotational lock between plugs. The rotational lock between the two bridge plugs decreases the required milling time. The mill will mill out the remaining portion of the upper plug and begin milling out the lower plug until the slips of the lower plug have been milled out. At this point, the lower plug will drop down the wellbore to the next bridge plug and the process is repeated until all of the bridge plugs have been removed from the wellbore.
Despite the above discussed improvements to bridge plugs, the milling time required to mill-out bridge plugs can vary greatly, especially for bridge plugs positioned below the most upper plug. As discussed above, the fracturing fluid pumped into the zone of interest often contains proppant. As a result a large amount of proppant may remain within the wellbore between two set bridge plugs. The amount of proppant present within the wellbore may vary depending on various factors such as the length of the perforated zone, the amount of under displacement or over displacement in the zone, the concentration of proppant in the fracturing fluid, or the amount of flow back used during the fracturing procedure. The presence of proppant within a zone may prevent the portion of an upper bridge plug from falling directly on top of a lower plug. Instead, the upper bridge plug may rest on proppant between the two plugs.
The proppant may prevent the profiles on the plugs from engaging and creating a rotational lock. Thus, the upper bridge plug is free to rotate on top of the proppant increasing the milling time required to mill out the plug. Mills used to remove a bridge plug from the wellbore, such as four or five bladed junk mills, usually include wash ports. Current designs of mills are concerned with effectively cutting through a set bridge plug and circulating the cuttings to the surface, but are not designed to fluidize and remove proppant located below a partially milled out bridge plug. The circulation of fluid from the mill wash ports in combination with the rotation of the upper bridge plug does seem to gradually remove the proppant from between the two plugs, but conventional milling blades are not efficient in removing the proppant from below a partially milled out bridge plug. This inefficiency may be due to the small amount of clearance between the bridge plug and the casing in combination with the location of wash ports being located around the perimeter of conventional mills. When a large amount of proppant is present it can take well over an hour for a conventional mill to cut through the remaining portion of the upper bridge plug and cut through the lower bridge plug until the slips have been removed dropping the lower bridge plug within the wellbore. This increased milling time increases the overall time and costs to remove each of the bridge plugs from the wellbore.
In light of the foregoing, it would be desirable to provide a system that provides fluid to fluidize and remove proppant from beneath at least a portion of a bridge plug. It would further be desirable to provide a wellbore mill having a central port adapted to fluidize and circulate proppant or sand from beneath a partially milled bridge plug.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.